Carbon dioxide 1 Carbon dioxide Carbon dioxide [[Image:Dry Ice Pellets Subliming.jpg Sample of solid carbon dioxide or "dry ice", pellets]] Structural formula of carbon [[File:Carbon-dioxide-3D-vdW.svg Spacefill model [[File:Carbon-dioxide-2D-dimensions.svg dioxide with a bond length]] of carbon dioxide]] [[Image:Carbon_dioxide_structure.png Ball and stick model of carbon dioxide]] Identifiers [1] CAS number 124-38-9 [2] PubChem 280 [3] ChemSpider 274 [4] UNII 142M471B3J [5] EC number 204-696-9 UN number 1013 [6] KEGG D00004 [7] MeSH Carbon+dioxide [8] ChEBI CHEBI:16526 RTECS number FF6400000 [9] ATC code V03 AN02 Beilstein Reference 1900390 Gmelin Reference 989 [10] 3DMet B01131 [11] Jmol-3D images Image 1 [12] Image 2 Properties Molecular formula CO 2 Molar mass 44.01 g mol−1 Exact mass 43.989829244 g mol−1 Appearance Colorless gas Odor Odorless Carbon dioxide 2 Density 1.562 g/mL (solid at 1 atm and −78.5 °C) 0.770 g/mL (liquid at 56 atm and 20 °C) 1.977 g/L (gas at 1 atm and 0 °C) Melting point -78 °C, 194.7 K, -109 °F (subl.) Boiling point -57 °C, 216.6 K, -70 °F (at 5.185 bar) Solubility in water 1.45 g/L at 25 °C, 100 kPa Acidity (pK ) 6.35, 10.33 a Refractive index (n ) 1.1120 D Viscosity 0.07 cP at −78 °C Dipole moment zero Structure Molecular shape linear Hazards NFPA 704 Related compounds Other anions Carbon disulfide Carbon diselenide Other cations Silicon dioxide Germanium dioxide Tin dioxide Lead dioxide Related carbon oxides Carbon monoxide Carbon suboxide Dicarbon monoxide Carbon trioxide Related compounds Carbonic acid Carbonyl sulfide [13] (what is this?) (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references Carbon dioxide (chemical formula CO ) is a naturally occurring chemical compound composed of two oxygen 2 atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state, as a trace gas at a concentration of 0.039% by volume. As part of the carbon cycle known as photosynthesis, plants, algae, and cyanobacteria absorb carbon dioxide, light, and water to produce carbohydrate energy for themselves and oxygen as a waste product.[14] But in darkness photosynthesis cannot occur, and during the resultant respiration small amounts of carbon dioxide are produced.[15] Carbon dioxide also is a by-product of combustion; emitted from volcanoes, hot springs, and geysers; and freed from carbonate rocks by dissolution. As of October 2010, carbon dioxide in the Earth's atmosphere is at a concentration of 388 ppm by volume.[16] Atmospheric concentrations of carbon dioxide fluctuate slightly with the change of the seasons, driven primarily by Carbon dioxide 3 seasonal plant growth in the Northern Hemisphere. Concentrations of carbon dioxide fall during the northern spring and summer as plants consume the gas, and rise during the northern autumn and winter as plants go dormant, die and decay. Taking all this into account, the concentration of CO grew by about 2 ppm in 2009.[17] Carbon dioxide is a 2 greenhouse gas as it transmits visible light but absorbs strongly in the infrared and near-infrared, before slowly re-emitting the infrared at the same or longer wavelengths as what was absorbed. Before the advent of human-caused release of carbon dioxide to the atmosphere, concentrations tended to increase with increasing global temperatures, acting as a positive feedback for changes induced by other processes such as orbital cycles.[18] There is a seasonal cycle in CO concentration associated primarily with the Northern Hemisphere 2 growing season.[19] Carbon dioxide has no liquid state at pressures below 5.1 standard atmospheres (520 kPa). At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below −78 °C (−108 °F; 195 K) and the solid sublimes directly to a gas above −78 °C. In its solid state, carbon dioxide is commonly called dry ice. CO is an acidic oxide: an aqueous solution turns litmus from blue to pink. It is the anhydride of carbonic acid, an 2 acid which is unstable in aqueous solution, from which it cannot be concentrated. In organisms carbonic acid production is catalysed by the enzyme, carbonic anhydrase. CO + H O H CO 2 2 2 3 CO is toxic in higher concentrations: 1% (10,000 ppm) will make some people feel drowsy.[20] Concentrations of 2 7% to 10% cause dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour.[21] Chemical and physical properties Carbon dioxide is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor. It can cause asphyxiation and irritation. When inhaled at concentrations much higher than usual atmospheric levels, it can produce a sour taste in the mouth and a stinging sensation in the nose and throat. These effects result from the gas dissolving in the mucous membranes and saliva, forming a weak solution of carbonic acid. This sensation can also occur during an attempt to stifle a burp after drinking a carbonated beverage. Amounts above 5,000 ppm are considered very unhealthy, and those above about 50,000 ppm (equal to 5% by volume) are considered dangerous to animal life.[22] At standard temperature and pressure, the density of carbon Carbon dioxide pressure-temperature phase diagram 3 showing the triple point and critical point of carbon dioxide dioxide is around 1.98 kg/m , about 1.5 times that of air. The carbon dioxide molecule (O=C=O) contains two double bonds and has a linear shape. It has no electrical dipole, and as it is fully oxidized, it is moderately reactive and is non-flammable, but will support the combustion of metals such as magnesium. Above , carbon dioxide changes directly from a solid phase to a gaseous phase through sublimation, or from gaseous to solid through deposition. Solid carbon dioxide is commonly called "dry ice", a generic trademark. It was first observed in 1825 by the French chemist Charles Thilorier. Dry ice is commonly used as a cooling agent, and it is relatively inexpensive. A convenient property for this purpose is that solid carbon dioxide sublimes directly into the gas phase, leaving no liquid. It can often be found in grocery stores and laboratories and is also used in the shipping industry. The largest non-cooling use for dry ice is blast cleaning. Carbon dioxide 4 Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about 518 kPa at −56.6 °C (see phase diagram, above). The critical point is 7.38 MPa at 31.1 °C.[23] Solid carbon dioxide, an amorphous glass-like solid, is known, although not at atmospheric pressure.[24] This form of glass, called carbonia, was produced by supercooling heated CO at extreme pressure (40–48 GPa or about 400,000 2 atmospheres) in a diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like silicon (silica glass) and germanium. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts back to gas when pressure is released. History Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the seventeenth century, the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (spiritus sylvestre). The properties of carbon dioxide were studied more thoroughly in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he called "fixed air." He observed that the fixed air was denser than air and Crystal structure of dry ice supported neither flame nor animal life. Black also found that when bubbled through an aqueous solution of lime (calcium hydroxide), it would precipitate calcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, English chemist Joseph Priestley published a paper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid (or oil of vitriol as Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of water in contact with the gas.[25] This was the invention of Soda water. Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday.[26] The earliest description of solid carbon dioxide was given by Charles Thilorier, who in 1834 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO .[27] 2 Isolation and production Carbon dioxide can be obtained by or from air distillation, however, this method is inefficient. A variety of chemical routes to carbon dioxide are known, such as the reaction between most acids and most metal carbonates. For example, the reaction between hydrochloric acid and calcium carbonate (limestone or chalk) is depicted below: 2 HCl + CaCO → CaCl + H CO 3 2 2 3 The carbonic acid (H CO ) then decomposes to water and CO .